1. Field of the Invention
The field of art to which this invention pertains is catalyst regeneration apparatus. More specifically, the present invention relates to a regeneration apparatus which is applicable for use in regenerating fluidizable catalytic cracking catalysts which have become spent by the deposition of coke on the catalysts in a hydrocarbon reaction zone.
2. Description of the Prior Art
In most regeneration processes presently employed the oxidation of coke from spent catalyst is done in a single-vessel regeneration apparatus containing one or more dense-phase beds of catalyst located in the bottom of the apparatus with a large dilute-phase disengaging space positioned above and in connection with a dense bed. In this type of regeneration process the dense-phase bed of catalyst is maintained in the bottom portion of the apparatus by limiting the superficial velocity of the incoming fresh regeneration gas to the transport velocity, that is, the velocity above which large amounts of catalyst would be carried out of the dense bed to the disengaging space. Typical velocities are therefore less than about 3 feet per second with 1.5 to 2.5 being the usual range. Provisions are made for recovering and returning to the dense bed any catalyst entrained in the flue gas effluent passing from the dense bed. This is generally carried out by passing this effluent flue gas containing entrained catalyst through separation means such as cyclone separation devices located in the disengaging space and returning separated catalyst to the same dense bed. Average residence time of the catalyst within the apparatus per pass through the apparatus is generally in the two to five minute range with 2 to 3 minutes being the more common, while the residence time of gas is generally within the range of 10 to 20 seconds. All of the regenerated catalyst is returned directly from the regeneration apparatus to the hydrocarbon reaction zone without additional passes through any part of the regeneration apparatus.
It is also general refinery practice to operate conventional regeneration apparatus in a manner to essentially preclude afterburning anywhere within the regeneration apparatus. The term "afterburning" as used herein and as generally understood by those skilled in the art shall mean the uncontrolled, unintentional, and generally incomplete oxidation of CO to CO.sub.2. Generally, afterburning occurs during periods of unsteady state operations or process "upsets." This prevention of significant afterburning is generally done by controlling the oxygen-containing gas stream introduced to such regeneration apparatus directly responsive to a rather small predetermined temperature differential between the flue gas outlet or the disengaging space and the dense bed of the regeneration apparatus. Excess oxygen within the regeneration apparatus is thus minimized thereby severely limiting CO afterburning to only that amount characterized by the small temperature differential.
Since the conversion of CO to CO.sub.2 is quite exothermic, this restricting of CO afterburning in conventional regeneration apparatus is done for the very practical reason of avoiding the damaging effects of excessively high temperatures in the upper disengaging space region of the regeneration apparatus where there is little catalyst present to act as a heat sink. This practice, as exemplified by Pohlenz U.S. Pat. Nos. 3,161,583 and 3,206,391, produces a small amount of oxygen in the flue gas, generally in the range of about 0.1 to 1% oxygen, results in the flue gas containing from about 7 to about 14 vol. % CO and limits the temperatures achieved in the regeneration apparatus to a maximum of about 1275.degree. F. Present industry practice is to direct the flue gas containing CO to the atmosphere or to a CO boiler where it is used as fuel to make steam.
Controlling the amount of fresh regeneration gas to permit a slight amount of afterburning and the once-through flow of catalyst through the regeneration apparatus essentially fixes the degree of catalyst regeneration, that is, the amount of residual coke on regenerated catalyst. Although it is widely known that the residual coke content on regenerated catalyst has a great influence on the conversion and product distribution obtained in the reaction zone, residual coke level on regenerated catalyst produced by present regeneration processes conducted in conventional regeneration apparatus is not an independent variable but is fixed for each regeneration apparatus design at a level typically from about 0.05 to about 0.4 wt. % carbon, and more often from about 0.15 to about 0.35 wt. % carbon.
By way of contrast, my invention centers around an apparatus for oxidizing coke and for the intentional, controlled, and essentially complete conversion of CO to CO.sub.2 within the apparatus. More specifically, the apparatus of my invention provides for coke oxidation and for essentially complete combustion within the apparatus of the CO produced and for the recovery within the apparatus of at least a portion of the heat of combustion of CO. This is distinguished from conventional regeneration apparatus in which afterburning is essentially precluded anywhere within the apparatus and in which no chemical heat of CO combustion is recovered within the apparatus. My invention recognizes the differences in the kinetics of coke oxidation and CO oxidation and provides separate regions within the regeneration apparatus for each to take place. Coke is oxidized primarily in a dense bed of fluidized catalyst in the spent-catalyst receiving chamber to produce regenerated catalyst and partially spent regeneration gas which are passed through a transfer conduit where essentially complete CO oxidation takes place and where heat of combustion is transferred to the regenerated catalyst passing through that zone. The resulting hot regenerated catalyst and spent regeneration gas are separated within a regenerated-catalyst receiving chamber and the regenerated catalyst is directed to a dense bed in the bottom portion of the regenerated-catalyst receiving chamber.
An internal regenerated-catalyst recycle means and an external regenerated-catalyst recycle means are provided to return hot regenerated catalyst from the transfer conduit and from the dense bed of catalyst in the regenerated-catalyst receiving chamber, respectively, to the dense bed of catalyst in the spent-catalyst receiving chamber to increase the catalyst residence time and the temperature in the spent-catalyst receiving chamber and hence the rate and extent of coke oxidation. The increased rate of reaction and catalyst residence time within the spent-catalyst receiving chamber result in regenerated catalyst having lower levels of residual coke. Additionally, the rate of CO burning in the transfer conduit is also increased because of the higher inlet temperature thereby producing lower CO concentrations in the spent regeneration gas leaving the apparatus. The remainder of the regenerated catalyst from the regenerated-catalyst receiving chamber is returned to the hydrocarbon reaction zone at a higher temperature than is produced in non-CO-burning regeneration apparatus which permits reduced feed preheat requirements.